The present inventions relates to gas sensors and to methods of making them.
Applicants are particularly, though not exclusively, interested in gas sensors for use in monitoring gases, such as acid gases, e.g. H.sub.2 S, NO.sub.2 and SO.sub.2, in connection for example with industrial process control or environmental protection.
It is well known that organic polymers may form the gas sensing material in gas sensors. Such organic materials include conducting polymers which are normally p-type semiconductors whose conductivities are changed when exposed to oxidising gases such as NO.sub.x or reducing gases such as NH.sub.3. Sensors using such materials have been based on both electrical techniques and optical techniques (e.g. surface plasmon resonance).
Various organic polymers, such as polyaniline, which appear to be suitable as gas sensing materials are commonly deliberately `doped` to improve the specificivity and/or sensitivity of the materials towards particular gases. Such `doping` also increases the electrical conductivity and facilitates the detection of change in conductivity of the polymer on exposure to and interaction with the gases being sensed; the change in conductivity being used as a measure of the concentration of the sensed gases.
With some organic polymers, such as polyaniline, in order for doping to be achieved the cationic part of the dopant can only be a hydrogen ion. In such a case the doped form of the polymer can be regarded as the protonated form.